Blue-chip shares took another tumble as fears over a reduction in US stimulus pulled the top-flight below the 6400 mark for the first time since the start of July.
It marks the latest episode in a topsy-turvy summer which saw the market rise on indications from London and Europe that central bank support was here to stay before sentiment was gnawed away by anxiety over the stance of the US Federal Reserve.
The FTSE 100 Index was down 62.6 points, or nearly 1%, to 6390.8 while the New York's Dow Jones Industrial Average was well into negative territory in early trading and European bourses were also moderately off.
On the currency markets the pound was little changed against the greenback at 1.57 US dollars but was up one cent against the single currency at 1.17 euros following comments by the German finance minister that Greece may need another bail-out.
The FTSE's fall below 6400 comes after it had climbed above the marker early in July following hints from the Bank of England that interest rates would remain low for some time to come and similar language from the European Central Bank.
But August has so far been an altogether different story, with details of Mark Carney's forward guidance on rates disappointing the City and growing "taper" talk over the likely actions of America's policymakers amid improving US economic data.
Miners have come under pressure on fears that the tapering off of quantitative easing in the world's largest economy could trigger a drop-off in global demand, particularly in emerging markets.
BHP Billiton was among the fallers, with a decline of more than 2%, or 50.5p, to 1873p, while Antofagasta slipped 17p to 895p.
Commodities trader Glencore Xstrata bucked the trend as investors returned to the stock in the wake of a big fall yesterday, when sentiment was hit by a £5 billion write-down on the value of its recent Xstrata acquisition.
Glencore shares were more than 2%, or 7.6p, higher at 304.8p, making it the session's strongest performer on the FTSE 100.
Meanwhile, ITV climbed 1.8p to 163.7p after a positive read-across from the results from Scottish commercial station STV, which posted a 5% hike in underlying pre-tax profits to £6.7 million and reported further momentum in its expanding digital and production operations.
STV was 10.5p higher at 184.5p as it revealed plans to resume shareholder dividend payments for the first time in seven years.
There was little in the way of major corporate news to excite traders, although Lloyds Banking Group was 0.1p higher at 73.9p after offloading its German insurance business Heidelberger Leben in a £250 million deal.
The taxpayer-backed bank will take a £330 million loss on the transaction but the removal of the insurer from its books will boost Lloyds' capital position by £400 million.
The biggest risers on the FTSE 100 were Glencore Xstrata, up 7.6p to 304.8p, Wolseley, up 70p to 3194p, WPP climbing 25p to 1191p and London Stock Exchange Group up 21p to 1566p.
The biggest fallers on the FTSE 100 were Intercontinental Hotels, down 133p to 1845p, Aberdeen Asset Management down 22.5p to 356.3p, HSBC off 22.2p at 682p and Hammerson down 13.9p to 488.1p.